Lincompex systems are well known in the telecommunications art. Such a digital Lincompex system is taught in U.S. Pat. No. 4,271,499 (the "'499 patent") entitled "Method and Apparatus for Digitally Implementing a Linked Compressor-Expander Telecommunications System" which issued Jun. 2, 1981, to Leveque, the Inventor of the present application. The '499 patent only teaches the transmitting of a voice signal over a Lincompex system. However, it has also been found that Lincompex systems can be utilized to transmit data having a complex waveform. Such a system that overcomes the inherent problem of data and envelope overlap (i.e., complex waveform) is fully disclosed in U.S. Pat. No. 4,907,217 (the "'217 patent"), also to Leveque, issued on Mar. 6, 1990, entitled "System and Method of Transmitting A Complex Waveform Over a Communication Channel Utilizing Lincompex Techniques".
In the voice transmission system, as disclosed in the '499 patent, a voice signal is transmitted using Lincompex techniques. This voice signal is band limited and exhibits a control tone 4 (according to the teachings of the '499 patent) which is also band limited and does not overlap the voice band 2, (see FIG. 2(a)).
FIGS. 1(a) and 1(b) illustrates the Lincompex System of the '499 patent.
In the modulator of FIG. 1(a) an input 20 of the Lincompex system modulator receives voice information for transmission. A control tone generator or envelope circuit 24 monitors the input voice signal. A compressor 22 compresses the introduced input voice signal. Compression is performed by dividing the signal by its envelope in pseudo-real time to produce a compressed voice signal. To develop the control tone, the system of FIG. 1(a) supplies the envelope signal developed from the output of the envelope detector 26 to a logarithmic (log) amplifier 28 which then develops a signal representative of the logarithm of the envelope signal. The output of this logarithmic amplifier 28 is supplied to a control terminal of a voltage control FM oscillator 30, which generates a frequency that varies about a center frequency F.sub.c in relation to the variation of the input voltage supplied to its control terminal from the logarithmic amplifier 28, to develop an envelope signal as an output of the control tone generator (envelope circuit) 24.
A summer 32 then sums the compressed voice signal developed at the output of the compressor 22 with the control tone developed at the output of the FM oscillator 30 to form a combined information signal. The summer 32, compressor 22 and control tone generator 24 form a Lincompex compressor LPXC.
In this speech transmission system, the combined information signal output from the summer 32 is provided to a transmitter 34 which transmits the signal over a desired transmission medium 36. In a typical embodiment, a single side band transmitter would normally transmit the modulated combined information signal across the airways in a known manner.
As illustrated in FIG. 1(b), the prior art Lincompex demodulator includes a receiving unit 15 which receives a modulated combined information signal from the transmission medium 36, which normally includes an antenna for receiving radio waves from the atmosphere, and supplies the received modulated combined information signal to a receiver 38. The receiver 38 demodulates the transmitted signal to produce a demodulated version of the combined information signal. Typically, this receiver 38 would be a single side band receiver which mixes the received modulated combined information signal with the channel frequency to produce a base band combined information signal.
To recover only the voice signal from such a combined voice/envelope information signal, a low pass filter 40 removes the voice information 2 of FIG. 2(a) from the combined information signal to recover the compressed voice signal containing only the voice information 2. This compressed voice signal is transmitted according to the Lincompex techniques at a substantially complete modulation of the transmitter 34 of FIG. 1(a). The compressed voice signal is passed through a fading regulator 58. The fading regulator 58 is a fading regulator similar to the one described in the '499 patent which removes any audio level variations not removed by the automatic gain control circuitry in the receiver 38. This compressed modified voice signal output from the fading regulator 58 is then expanded to produce the necessary dynamic range for the recovered voice signal to be supplied at the output 60.
Accordingly, an expander 42, similar to the one described in the '499 patent, is utilized which essentially multiplies the compressed modified voice signal developed at the output of the fading regulator 58 by an envelope signal which is recovered by a control tone conversion circuit 44.
The control tone conversion circuit 44 comprises a band pass filter 46 which recovers only the control tone which is represented by the control tone information signal 4 in FIG. 2(a). The control tone originally developed by the FM oscillator 30 of FIG. 1(a) is then passed through a frequency discriminator 48. The frequency discriminator 48 measures the instantaneous frequency of the control tone and produces a voltage level representative of this measured frequency. In this case, the voltage level also represents the logarithm of the original envelope signal. The anti-logarithm circuit 50 is utilized to recover the original envelope signal. This logarithmic signal is then supplied to an anti-logarithm circuit 50. The anti-logarithm circuit 50 is utilized to recover the original envelope developed by the envelope detector 26 of FIG. 1(a). This original envelope signal is used by the expander 42 to recover the original voice signal by expanding the compressed voice signal to provide the original signal to the output 60. This control tone conversion circuit 44 is similar to the control tone conversion circuit described in the '499 patent. The control tone conversion circuit 44, low pass filter 40, fading regulator 58, and expander 42 form a Lincompex expander LPXE.
With respect to the transmission of data, the Lincompex techniques used may or may not be different from the transmission of voice signals due to the complex nature of the data waveform. This technique is fully discussed in the '217 patent.
The above described Lincompex systems have been described with respect to single side band transmission and reception. However, it has also been found that Lincompex signals can be transmitted and received utilizing frequency modulation. Such a system is fully disclosed in U.S. Pat. No. 5,058,202 (the "'202 patent"), also to Leveque, issued on Oct. 15, 1991, entitled "System and Method of Transmitting And Receiving A Lincompex Modulated Signal Over A Communication Channel Utilizing Frequency Modulation Techniques".